去年___、今年の夏は過ごしやすい。
Q. Which phrase completes 'compared to last year'?
Translation
Compared to last year, this summer is comfortable.
Explanation
〜に比べて is the set comparative phrase: 去年に比べて. The thing compared against takes に (or と), never が, and を would require 比べる as a full transitive clause.